CYA test against a standard curve

[Panaxdave]

I teach biology at a local community college and, because of that, have access to some older spectrophotometers (Spec 20s for those of you that have used them). I often times use them to get growth curves for bacteria. Is there any reason why I couldn't take some standardized cya solutions and plot a standard curve for the test? I could then just measure mine against the curve instead of trying to see or not see that dang black dot. Anyone know what wavelength might be best to use? Also, by looking at the cya test tube in the tf100, I'm assuming my curve will not be linear. I would create a pretty large curve and not extrapolate outside. Anyone tried this?

 

[JoyfulNoise]

I teach biology at a local community college and, because of that, have access to some older spectrophotometers (Spec 20s for those of you that have used them). I often times use them to get growth curves for bacteria. Is there any reason why I couldn't take some standardized cya solutions and plot a standard curve for the test? I could then just measure mine against the curve instead of trying to see or not see that dang black dot. Anyone know what wavelength might be best to use? Also, by looking at the cya test tube in the tf100, I'm assuming my curve will not be linear. I would create a pretty large curve and not extrapolate outside. Anyone tried this?

The data points would be fit with an exponential function as turbidity (light scattering) is exponential with optical path length. Typically in a nephlometry test, the detector is at 90 deg to the light source as you only want to look at the intensity of scattered light. If your spectrometer can only due in-line detection, you might not have enough path length in a standard cuvette to make a reasonable curve. “White” light is typically used as a source for turbidity measurements.

Worth a try, but you might have to run a bunch of parametric testing to see if your setup can give you the results you need.

Standards can be difficult to make as CYA will change the pH of distilled water by quite a lot. You’ll need to create a properly buffered water solution that simulates pool water as the reagent (R-0013) is designed to adjust the pH of the resulting mixture so that it is acidic. If the standard sample already starts off in an acidic condition, that might affect reagent performance.

 

[Panaxdave]

The data points would be fit with an exponential function as turbidity (light scattering) is exponential with optical path length. Typically in a nephlometry test, the detector is at 90 deg to the light source as you only want to look at the intensity of scattered light. If your spectrometer can only due in-line detection, you might not have enough path length in a standard cuvette to make a reasonable curve. “White” light is typically used as a source for turbidity measurements.

Worth a try, but you might have to run a bunch of parametric testing to see if your setup can give you the results you need.

Standards can be difficult to make as CYA will change the pH of distilled water by quite a lot. You’ll need to create a properly buffered water solution that simulates pool water as the reagent (R-0013) is designed to adjust the pH of the resulting mixture so that it is acidic. If the standard sample already starts off in an acidic condition, that might affect reagent performance.

Thanks for that reply! Spec 20s are in line so that'll be an issue. Not sure what our Chem dept has but I'll check. I've got some time on my hands since it's the summer semester, so I might play around with some simulated pool water at various CYA levels and see what I get. As usual, and the reason I ask the question, there are always issues you'll run into when you think "Ah, this should be easy".

 

[JoyfulNoise]

Do some checking around with Hach, LaMotte and other test suppliers. There may be a color indicator for CYA available. The CYA test strips use a pH change reaction between CYA and a reagent. The only reason why it’s terrible in strips is because the color change is very subtle and only becomes noticeable to the human eye when the CYA concentration differences are large. However, for a spectrometer, you might be able to see the absorption differences easier.

 

[mopar440_6]

If you're willing to throw a couple bucks at this experiment, you can buy 50ppm standard solution from TF Test Kits.

http://tftestkits.net/R-7065-CYA-Standard-50-ppm-2-ounces-p97.html

 

[woodyp]

This is Deep End stuff for me!